Napoleon I, emperor of France, was sentinto exile on the remote island of St. Helenaby the British after his defeat at the Battleof Waterloo in 1815. St. Helena was hot,unsanitary, and rampant with disease.There, Napoleon was confined to a largereconstructed agricultural building knownas Longwood House. Boredom andunhealthy living conditions gradually tooktheir toll on Napoleon’s mental andphysical state. He began suffering fromsevere abdominal pains and experiencedswelling of the ankles and generalweakness of his limbs. From the fall of1820, Napoleon’s health began todeteriorate rapidly until death arrived onMay 5, 1821. An autopsy concluded thecause of death to be stomach cancer.

It was inevitable that dying underBritish control, as Napoleon did, wouldbring with it numerous conspiratorialtheories to account for his death. One of

the more fascinating inquiries wasconducted by a Swedish dentist, SvenForshufvud, who systematically correlatedthe clinical symptoms of Napoleon’s lastdays to those of arsenic poisoning. ForForshufvud, the key to unlocking the causeof Napoleon’s death rested withNapoleon’s hair. Forshufvud arranged tohave Napoleon’s hair measured for arseniccontent by neutron activation analysis andfound it consistent with arsenic poisoningover a lengthy period of time.Nevertheless, the cause of Napoleon’sdemise is still a matter for debate andspeculation. Other Napoleon hairs havebeen examined and found to be low inarsenic content. Some question whetherNapoleon even had clinical symptomsassociated with arsenic poisoning. Intruth, forensic science may never be ableto answer the question—what killedNapoleon?

What Killed Napoleon?

Key Terms

alpha ray

atomic mass

atomic number

beta ray

continuous spectrum

density-gradient tube

electron

electron orbital

emission spectrum

excited state

gamma ray

isotope

line spectrum

mineral

neutron

nucleus

proton

pyrolysis

radioactivity

388 CHAPTER 11

Considering that most materials originate from the earth’s crust, it is notsurprising that they are rarely obtained in pure form; instead, they includenumerous elemental impurities that usually must be eliminated throughindustrial processing. However, it is usually not economically feasible tocompletely exclude all such minor impurities, especially if their presencehas no effect on the appearance or performance of the final product. Forthis reason, many manufactured products, and even most natural materi-als, contain small quantities of elements present in concentrations of lessthan 1 percent, which are known as trace elements.

Forensic Analysis of MetalsFor the criminalist, the presence of trace elements is particularly useful be-cause they provide “invisible” markers that may establish the source of amaterial or at least provide additional points for comparison. For example,comparison of trace elements present in two metallic specimens may pro-vide data on their source or origin. Table 11–1 illustrates how two types ofbrass alloys can readily be distinguished by their elemental composition.

Forensic investigators have examined the evidential value of trace ele-ments present in metallic objects as well as paint, soil, glass, and fibers.One example is the examination of the bullet and bullet fragments recov-ered after the assassination of President John F. Kennedy.

Evidence in the Assassination of President KennedyEver since President Kennedy was killed in 1963, questions have lingeredabout whether Lee Harvey Oswald was part of a conspiracy to assassinatethe president, or a lone assassin. The Warren Commission, the officialgovernment body appointed to investigate the shooting, concluded thatOswald acted alone. This conclusion was criticized widely at the time it wasreported, and has been a source of controversy since that day.

Reconstructing the Assassination In arriving at its conclusions, the WarrenCommission reconstructed the crime as follows: From a hidden positionon the sixth floor of the Texas School Book Depository building, Oswaldfired three shots from behind the president. The president was struck bytwo bullets, with one bullet totally missing the president’s limousine. One

■ Describe the usefulness of trace elements for forensiccomparison of various types of physical evidence

■ Define and distinguish protons, neutrons, and electrons

■ Define and distinguish atomic number and atomic massnumber

■ Explain the concept of an isotope

■ Understand how elements can be made radioactive

■ List the most useful examinations for performing aforensic comparison of paint

■ Distinguish continuous and line emission spectra

■ Understand the parts of a simple emissionspectrograph

■ Appreciate the phenomenon of how an atom absorbsand releases energy in the form of light

■ Describe proper collection and preservation of forensicpaint evidence

■ List the important forensic properties of soil

■ Describe proper collection of soil evidence

Learning ObjectivesAfter studying this chapter you should be able to:

Trace Evidence II: Metals, Paint, and Soil 389

bullet hit the president in the back, exited his throat, and went on to strikeTexas Governor John Connally, who was sitting in a jump seat in front ofthe president. The bullet hit Connally in his back, exited his chest, struckhis right wrist, and temporarily lodged in his left thigh. This bullet was laterfound on the governor’s stretcher at the hospital. A second bullet in theskull fatally wounded the president.

Table 11–1 Elemental Analysis of Brass Alloys

High-Tensile Brass Manganese Brass Element

(percentage) (percentage)

Copper 57.0 58.6

Aluminum 2.8 1.7

Zinc 35.0 33.8

Manganese 2.13 1.06

Iron 1.32 0.90

Nickel 0.48 1.02

Tin 0.64 1.70

Lead 0.17 0.72

Silicon 0.08 Nil

Source: R. L. Williams, “An Evaluation of the SEM with X-Ray Microanalyzer Accessory forForensic Work,” in Scanning Electron Microscopy/1971, O. Johari and I. Corvin, eds. (Chicago:IIT Research Institute, 1971), p. 541.

FIGURE 11–1 President John F. Kennedy, Governor John Connally of Texas, and Mrs.Jacqueline Kennedy ride through Dallas moments before the assassination. CourtesyCorbis/Bettmann

390 CHAPTER 11

In a room at the Texas School Book Depository, a 6.5-millimeterMannlicher-Carcano military rifle was found with Oswald’s palm print onit. Also found were three spent 6.5-millimeter Western Cartridge Co./Mannlicher-Carcano (WCC/MC) cartridge cases. Oswald, an employee ofthe depository, had been seen there that morning and also a few minutesafter the assassination, disappearing soon thereafter. He was apprehendeda few miles from the depository nearly two hours after the shooting.

Questions and Criticisms Critics of the Warren Commission have longargued that evidence proves that Oswald did not act alone. Eyewitness ac-counts and acoustical data interpreted by some experts have been used tocontend that a second shooter fired at the president from a region in frontof the limousine (the so-called grassy knoll). Furthermore, critics arguethat the Warren Commission’s reconstruction of the crime assumed thatonly one bullet caused both the president’s throat wound and GovernorConnally’s back wound. Critics contend that such damage would have de-formed and mutilated a bullet. Instead, the recovered bullet showed someflattening, no deformity, and only about 1 percent weight loss.

In 1977 the U.S. House of Representatives Select Committee on As-sassinations requested that the bullet taken from Connally’s stretcher—along with bullet fragments recovered from the car and various woundareas—be examined for trace element levels. Because lead alloys used tomanufacture bullets contain an assortment of trace elements, analysis ofthose trace elements provides a possible means for characterizing leadbullets. For example, antimony is often added to lead as a hardeningagent; copper, bismuth, and silver are other trace elements commonlyfound in bullet lead.

Investigators compared the antimony and silver content of the bulletand bullet fragments recovered after the assassination. Previous studieshad amply demonstrated that the levels of these two elements areparticularly important for characterizing WCC/MC bullets. Bullet leadfrom this type of ammunition ranges in antimony concentration from 20 to1,200 parts per million (ppm) and 5 to 15 ppm in silver content.

As can be seen in Table 11–2, the samples designated Q1 (the Connallystretcher bullet) and Q9 (bullet fragments from Connally’s wrist) are indis-tinguishable from one another in antimony and silver content. The samples

Table 11–2 Antimony and Silver Concentrations in the KennedyAssassination Bullets

Silver Antimony

Sample (parts per million)a (parts per million) Sample Description

Q1 8.8 � 0.5 833 � 9 Connally stretcher bullet

Q9 9.8 � 0.5 797 � 7 Fragments from Connally’s wrist

Q2 8.1 � 0.6 602 � 4 Large fragment from car

Q4, 5 7.9 � 0.3 621 � 4 Fragments from Kennedy’s brain

Q14 8.2 � 0.4 642 � 6 Small fragments found in car

aOne part per million equals 0.0001 percent.

Source: Reprinted with permission from V. P. Guinn, “JFK Assassination: Bullet Analyses,”Analytical Chemistry 51 (1979): 484 A. Copyright 1979, American Chemical Society.

neutronA particle with no electrical

charge that is one of the basic

structures in the nucleus of an

atom.

electronA negatively charged particle

that is one of the fundamental

structural units of the atom.

protonA positively charged particle

that is one of the basic

structures in the nucleus of an

atom.

Trace Evidence II: Metals, Paint, and Soil 391

designated Q4 and Q5 (fragments from Kennedy’s brain) are indistin-guishable in antimony and silver content from samples Q2 and Q14 (frag-ments recovered from two different areas in the car). However, all four ofthose samples are different from Q1 and Q9.

Conclusions From studying these results we can derive the followingconclusions:

1. There is evidence of only two bullets—one composed of 815 ppm anti-mony and 9.3 ppm silver, the other composed of 622 ppm antimony and8.1 ppm silver.

2. Both bullets have a composition highly consistent with WCC/MC bul-let lead, although other sources cannot be ruled out entirely.

3. The bullet found on the Connally stretcher also damaged Connally’swrist. The absence of bullet fragments from the back wounds ofKennedy and Connally prevented any effort at linking these wounds tothe stretcher bullet.

None of these conclusions can verify absolutely the Warren Commis-sion’s reconstruction of the assassination, but the results are at least con-sistent with the commission’s findings. The analyses on the Kennedyassassination bullets used a method known as neutron activation analysis,one of several forensic techniques we will examine in this chapter.

Atomic StructureTo understand the principle behind neutron activation analysis, one mustfirst understand the fundamental structure of the atom. Each atom is com-posed of elementary particles that are collectively known as subatomic par-ticles. The most important subatomic particles are the proton, electron,and neutron.

The properties of the proton, neutron, and electron are summarized inthe following table:

Particle Symbol Relative Mass Electrical Charge

Proton P 1 �

Neutron n 1 0

Electron e 1/1837 �

As you can see, the masses of the proton and neutron are each about 1,837times the mass of an electron. The proton has a positive electrical charge; theelectron has a negative charge equal in magnitude to that of the proton; andthe neutron is a neutral particle with neither a positive nor a negative charge.

A popular descriptive model of the atom, and the one that will beadopted for the purpose of this discussion, pictures an atom as consistingof electrons orbiting a central nucleus composed of protons andneutrons—an image that is analogous to our solar system, in which theplanets revolve around the sun.1 To maintain a zero net electrical charge,the number of protons in the nucleus must always equal the number ofelectrons in orbit around the nucleus.

With this knowledge, we can describe the atomic structure of the ele-ments. For example, hydrogen has a nucleus consisting of one proton andno neutrons, and it has one orbiting electron. Helium has a nucleus com-prising two protons and two neutrons, with two electrons in orbit aroundthe nucleus (see Figure 11–2).

nucleusThe core of an atom,

consisting of protons and

neutrons.

392 CHAPTER 11

1P

Hydrogen

2P

2n

Helium

FIGURE 11–2 The atomic structuresof hydrogen and helium.

The behavior and properties that distinguish one element from anothermust be related to the differences in the atomic structure of each element. Onesuch distinction is that each element possesses a different number of protons.This number is called the atomic number of the element. As we look back atthe periodic table on p. 119, we see that the elements are numbered consecu-tively. Those numbers represent the atomic number or number of protons as-sociated with each element. An element is therefore a collection of atoms thatall have the same number of protons. Thus, each atom of hydrogen has oneand only one proton, each atom of helium has 2 protons, each atom of silverhas 47 protons, and each atom of lead has 82 protons in its nucleus.

Isotopes and RadioactivityAlthough the atoms of a single element must have the same number ofprotons, nothing prevents them from having different numbers of neutrons.The total number of protons and neutrons in a nucleus is known as theatomic mass number. Atoms with the same number of protons but differingsolely in the number of neutrons are called isotopes.

For example, hydrogen consists of three isotopes: ordinary hydrogen,which has one proton and no neutrons in its nucleus, and two other iso-topes called deuterium and tritium. Deuterium (or heavy hydrogen) alsohas one proton, but contains one neutron as well. Tritium has one protonand two neutrons in its nucleus.

Therefore, all the isotopes of hydrogen have an atomic number of 1 butdiffer in their atomic mass numbers. Hydrogen has an atomic mass of 1,deuterium a mass of 2, and tritium a mass of 3. Ordinary hydrogen makesup 99.98 percent of all the hydrogen atoms found in nature. The atomicstructures of these isotopes are shown in Figure 11–3.

Like hydrogen, most elements have two or more isotopes. Tin, for ex-ample, has ten isotopes. Many of these isotopes are quite stable, and theisotopes of any one element have indistinguishable properties. Others,however, are not as stable and decompose over time by a process knownas radioactive decay. Radioactivity is the emission of high-energy

Hydrogen Deuterium Tritium

1P1P

1n

1P

2n

FIGURE 11–3 Isotopes of hydrogen.

radioactivityThe emission of high-energy

subatomic particles that

accompanies the spontaneous

disintegration of unstable

nuclei.

isotopeAn atom differing from another

atom of the same element in

the number of neutrons it has

in its nucleus.

atomic massThe sum of the number of

protons and neutrons in the

nucleus of an atom.

atomic numberThe number of protons in the

nucleus of an atom.

Trace Evidence II: Metals, Paint, and Soil 393

subatomic particles that accompanies the spontaneous disintegration ofunstable nuclei.

Radioactivity is composed of three types of radiation: alpha rays, betarays, and gamma rays. Alpha rays are helium atoms stripped of their or-biting electrons; thus, they are positively charged particles. Each alpha rayparticle has a mass approximately four times that of a hydrogen atom. Betarays are electrons, and gamma rays are a form of electromagnetic radia-tion similar to X-rays (discussed in Chapter 4), but of a higher frequencyand energy. Fortunately, most naturally occurring isotopes are not ra-dioactive, and those that are—radium, uranium, and thorium—are foundin such small quantities in the earth’s crust that their radioactivity presentsno hazard to human survival.

When an atom is bombarded with neutrons, some neutrons are cap-tured to form new isotopes. This is what happens in a nuclear reactor.A nuclear reactor is simply a source of neutrons that bombard the atomsof a specimen, thereby creating radioactive isotopes. The nucleus of anatom that has captured one or more neutrons is said to be activated, and itoften begins to decompose immediately, emitting radioactivity.

Neutron Activation AnalysisForensic chemists can characterize the trace elements in a specimen bybombarding it with neutrons and measuring the energy of the gamma raysemitted by the activated isotopes. The gamma rays of each element are as-sociated with characteristic energy values, and thus exhibit unique levelsof energy. This technique, known as neutron activation analysis, is de-picted in Figure 11–4. Once an element has been identified, its concentra-tion can be measured by the intensity of its gamma-ray radiation; theintensity of the radiation is directly proportional to the concentration of theelement in a specimen.

The major advantage of neutron activation analysis is that it provides anondestructive method for identifying and quantitating trace elements. Amedian detection sensitivity of one-billionth of a gram (1 nanogram) makes

gamma rayA high-energy form of

electromagnetic radiation.

beta rayRadiation composed of

electrons.

alpha rayRadiation composed of helium

atoms minus their orbiting

electrons.

FIGURE 11–4 The neutronactivation process requiresthe capture of a neutron bythe nucleus of an atom. Thenew atom is now radioactiveand emits gamma rays. Adetector permitsidentification of theradioactive atoms present bymeasuring the energies andintensities of the gammarays emitted.

Atom

n

n

n

Neutron

Neutronsbombardspecimen

Atom

n

n

Neutron

Energy

Inte

nsity

Energy

Inte

nsity

Energy

Inte

nsity

Detector measures the energies and intensities of the gamma rays

Multichannel analyzer

Each element is associated with a characteristic energy value. Intensity indicatesthe element concentration in the specimen

Atom

Gamma Rays

Gamma Rays

394 CHAPTER 11

neutron activation analysis one of the most sensitive methods available forquantitative detection of many elements. Further, neutron activation cansimultaneously analyze twenty to thirty elements. A major drawback to thetechnique is its expense. Only a handful of crime laboratories have accessto a nuclear reactor; in addition, sophisticated analyzers are needed to de-tect and discriminate gamma-ray emissions.

Neutron activation has been used to characterize trace elements in met-als, drugs, paint, soil, gunpowder residues, and hair. A typical illustrationof its application occurred during the investigation of a theft of copper tele-graphic wires in Canada. Four lengths of copper wire (A1, A2, A3, A4) foundat the scene of the theft were compared by neutron activation with a lengthof copper wire (B) seized at a scrap yard and suspected of being stolen. Allwere bare, single-strand wire with the same general physical appearanceand a diameter of 0.28 centimeter.

Prior experiments had revealed that significant variations could be ex-pected in the concentration levels of the trace elements selenium, gold, an-timony, and silver for wires originating from different sources. Analystscompared these elements present in the wire involved in the theft. Afterexposing the wires to neutrons in a nuclear reactor, neutron activationanalysis revealed a match between A1 and B that was well within experi-mental error (see Table 11–3). The findings suggested a common origin ofthe control and suspect wires.

Key Points

• Trace elements are small quantities of elements present in concentrationsof less than 1 percent. They provide “invisible” markers that may establishthe source of a material or provide additional points for comparison.

• The three most important subatomic particles are the proton, neu-tron, and electron. The proton has a positive electrical charge, theneutron has no electrical charge, and the electron has a negative elec-trical charge.

• Atomic number indicates the number of protons in the nucleus of anatom. Atomic mass refers to the total number of protons and neutronsin a nucleus.

Table 11–3 Concentration of Trace Elements in Copper Wire

Selenium Gold Antimony Silver

Control Wire

A1 2.4 0.047 0.16 12.7

A2 3.5 0.064 0.27 17.2

A3 2.6 0.050 0.20 13.3

A4 1.9 0.034 0.21 12.6

Suspect Wire

B 2.3 0.042 0.15 13.0

Note: Average concentration measured in parts per million.

Source: R. K. H. Chan, “Identification of Single-Stranded Copper Wires by NondestructiveNeutron Activation Analysis.” Journal of Forensic Sciences 17 (1972): 93. Reprinted bypermission of the American Society for Testing and Materials, copyright 1972.

Trace Evidence II: Metals, Paint, and Soil 395

• An isotope is an atom differing from other atoms of the same elementin the number of neutrons in its nucleus.

• Radioactivity is the emission of high-energy subatomic particles thataccompanies the spontaneous disintegration of the nuclei of unstableisotopes. The three types of radiation are alpha rays, beta rays, andgamma rays.

• In neutron activation analysis, a sample is bombarded with neutronsand the energy of the gamma rays emitted by the activated isotopes ismeasured. The gamma rays of each element are associated with char-acteristic energy values that helps identify the specific element thatproduces them.

Forensic Examination of PaintOur environment contains millions of objects whose surfaces are painted.Thus paint, in one form or another, is one of the most prevalent types ofphysical evidence received by the crime laboratory.

Paint as physical evidence is perhaps most frequently encountered inhit-and-run and burglary cases. For example, a chip of dried paint or apaint smear may be transferred to the clothing of a hit-and-run victim onimpact with an automobile, or paint smears could be transferred onto atool during a burglary. Obviously, in many situations a transfer of paintfrom one surface to another could impart an object with an identifiableforensic characteristic.

In most circumstances, the criminalist must compare two or morepaints to establish their common origin. For example, such a comparisonmay associate an individual or a vehicle with the crime site. However, thecriminalist need not be confined to comparisons alone. Crime laboratoriesoften help identify the color, make, and model of an automobile by exam-ining small quantities of paint recovered at an accident scene. Such re-quests, normally made in hit-and-run cases, can lead to the apprehensionof the responsible vehicle.

Composition of PaintPaint is composed of a binder and pigments, as well as other additives, alldissolved or dispersed in a suitable solvent. Pigments impart color and hid-ing (or opacity) to paint and are usually mixtures of different inorganic andorganic compounds added to the paint by the manufacturer to producespecific colors and properties. The binder is a polymeric substance thatprovides the support medium for the pigments and additives. After painthas been applied to a surface, the solvent evaporates, leaving behind ahard polymeric binder and any pigments that are suspended in it.

One of the most common types of paint examined in the crime labora-tory is finishes from automobiles. Manufacturers apply a variety of coat-ings to the body of an automobile; this adds significant diversity toautomobile paint and contributes to the forensic significance of automo-bile paint comparisons. The automotive finishing system for steel usuallyconsists of at least four organic coatings:

Electrocoat Primer The first layer applied to the steel body of a car isthe electrocoat primer. The primer, consisting of epoxy-based resins, is

396 CHAPTER 11

electroplated onto the steel body of the automobile to provide corrosion re-sistance. The resulting coating is uniform in appearance and thickness. Thecolor of these primers ranges from black to gray.

Primer Surfacer Originally responsible for corrosion control, thesurfacer usually follows the electrocoat layer and is applied before thebasecoat. Primer surfacers are epoxy-modified polyesters or urethanes.The function of this layer is to completely smooth out and hide any seamsor imperfections, because the basecoat will be applied on this surface. Thislayer is highly pigmented. Color pigments are used to minimize color con-trast between primer and topcoats. For example, a light gray primer maybe used under pastel shades of a colored topcoat; a red oxide may be usedunder a dark-colored topcoat.

Basecoat The next layer of paint on a car is the basecoat or colorcoat.This layer provides the color and aesthetics of the finish and represents the“eye appeal” of the finished automobile. The integrity of this layer dependson its ability to resist weather, UV radiation, and acid rain. Most commonly,an acrylic-based polymer comprises the binder system of basecoats. Inter-estingly, the choice of automotive pigments is dictated by toxic and envi-ronmental concerns. Thus, the use of lead, chrome, and other heavy-metalpigments has been abandoned in favor of organic-based pigments. Thereis also a growing trend toward pearl luster or mica pigments. Micapigments are coated with layers of metal oxide to generate interferencecolors. Also, the addition of aluminum flakes to automotive paint impartsa metallic look to the paint’s finish.

Clearcoat An unpigmented clearcoat is applied to improve gloss, dura-bility, and appearance. Most clearcoats are acrylic based, but polyurethaneclearcoats are increasing in popularity. These topcoats provide outstand-ing etch resistance and appearance.

Microscopic Examination of PaintThe microscope has traditionally been and remains the most important in-strument for locating and comparing paint specimens. Considering thethousands of paint colors and shades, it is quite understandable why color,more than any other property, imparts paint with its most distinctive foren-sic characteristics. Questioned and known specimens are best comparedside by side under a stereoscopic microscope for color, surface texture, andcolor layer sequence (see Figure 11–5).

The importance of layer structure for evaluating the evidential signifi-cance of paint evidence cannot be overemphasized. When paint specimenspossess colored layers that match in number and sequence of colors, theexaminer can begin to relate the paints to a common origin. How many lay-ers must be matched before the criminalist can conclude that the paintscome from the same source? There is no one accepted criterion. Much de-pends on the uniqueness of each layer’s color and texture, as well as thefrequency with which the particular combination of colors under investi-gation is observed. Because no books or journals have compiled this typeof information, the criminalist is left to his or her own experience andknowledge when making this decision.

Unfortunately, most paint specimens do not have a layer structure of suf-ficient complexity to allow them to be individualized to a single source, nor isit common to have paint chips that can be physically fitted together to provecommon origin, as shown in Figure 11–6. However, the diverse chemicalcomposition of modern paints provides additional points of comparison

Trace Evidence II: Metals, Paint, and Soil 397

between specimens. Specifically, a thorough comparison of paint mustinclude a chemical analysis of the paint’s pigments, its binder composition,or both.

Analytical Techniques Used in Paint ComparisonThe wide variation in binder formulations in automobile finishes providessignificant information. More important, paint manufacturers make auto-mobile finishes in hundreds of varieties; this knowledge is most helpful to

FIGURE 11–5 A stereoscopic microscope comparison of two automotive paints. Thequestioned paint on the left has a layer structure consistent with the contol paint on theright. Courtesy Leica Microsystems, Inc., Buffalo, N.Y., www.leica-microsystems.com

FIGURE 11–6 Paint chip 1 was recovered from the scene of a hit-and-run. Paint chip 2was obtained from the suspect vehicle. Courtesy New Jersey State Police

398 CHAPTER 11

the criminalist who is trying to associate a paint chip with one car as dis-tinguished from the thousands of similar models that have been producedin any one year. For instance, there are more than a hundred automobileproduction plants in the United States and Canada. Each can use one paintsupplier for a particular color or vary suppliers during a model year. Al-though a paint supplier must maintain strict quality control over a paint’scolor, the batch formulation of any paint binder can vary, depending on theavailability and cost of basic ingredients.

Characterization of Paint Binders An important extension of the applicationof gas chromatography to forensic science is the technique of pyrolysis gaschromatography. Many solid materials commonly encountered as physicalevidence—for example, paint chips, fibers, and plastics—cannot be readilydissolved in a solvent for injection into the gas chromatograph. Thus, undernormal conditions these substances cannot be subjected to gas chromato-graphic analysis. However, materials such as these can be heated, orpyrolyzed, to high temperatures (500–1000°C) so that they will decomposeinto numerous gaseous products. Pyrolyzers permit these gaseous productsto enter the carrier gas stream, where they flow into and through the GCcolumn. The pyrolyzed material can then be characterized by the patternproduced by its chromatogram, or pyrogram.

Pyrolysis gas chromatography is particularly invaluable for distin-guishing most paint formulations. In this process, paint chips as small as20 micrograms are decomposed by heat into numerous gaseous productsand are sent through a gas chromatograph.

As shown in Figure 11–7, the polymer chain is decomposed by a heatedfilament, and the resultant products are swept into and through a gas chro-matograph column. The separated decomposition products of the polymeremerge and are recorded. The pattern of this chromatogram or “pyrogram”distinguishes one polymer from another. The result is a pyrogram that is

pyrolysisThe decomposition of organic

matter by heat.

Carrier gas

Pyrolyzer

Column

Detector

Pyrogram

FIGURE 11–7 Schematic diagram of pyrolysis gas chromatography.

Trace Evidence II: Metals, Paint, and Soil 399

sufficiently detailed to reflect the chemical makeup of the binder. Figure 11–8illustrates how the patterns produced by paint pyrograms can differentiateacrylic enamel paints removed from two different automobiles.

Infrared spectrophotometry is still another analytical technique thatprovides information about the binder composition of paint.2 Bindersselectively absorb infrared radiation to yield a spectrum that is highlycharacteristic of a paint specimen.

Characterization of Paint Pigments The elements that constitute the inorganic pigments of paints can be identified by a variety of techniques:

Time (minutes)

2 4 6 8 10 12

(a)

Time (minutes)

2 4 6 8 10 12

(b)

FIGURE 11–8 Paint pyrograms ofacrylic enamel paints. (a) Paint from aFord model and (b) paint from aChrysler model. Courtesy Varian Inc.,Palo Alto, Calif.

400 CHAPTER 11

emission spectroscopy, neutron activation analysis, and X-ray spectroscopy(pp. 252–253). The emission spectrograph for example, can simultaneouslydetect fifteen to twenty elements in most automobile paints. Some of theseelements are relatively common to all paints and have little forensic value;others are less frequently encountered and provide excellent points of com-parison between paint specimens.

We saw in Chapter 5 that organic compounds can be characterized byselective absorption of ultraviolet, visible, or infrared radiation. Equallysignificant to the forensic chemist is the knowledge that elements also se-lectively emit and absorb light. These observations form the basis of emis-sion spectroscopy.

When sunlight or the light from an incandescent bulb is passedthrough a prism, a range of rainbow colors is produced. The resulting dis-play of colors is called an emission spectrum. This type of emission spec-trum is called a continuous spectrum because all the colors merge orblend into one another to form a continuous band.

Not all light sources, however, produce a continuous spectrum. For ex-ample, if the light from a sodium lamp, a mercury arc lamp, or a neon lightis passed through a prism, the resultant spectrum consists not of a contin-uous band, but of several individual colored lines separated by darkspaces. Here, each line represents a definite wavelength or frequency oflight that is separate and distinct from all others in the spectrum. This typeof spectrum is called a line spectrum. Figure 11–9 shows the line spectraof three elements. If a solid or liquid is vaporized and “excited” by expo-sure to high temperature, each element that is present emits light com-posed of select frequencies that are characteristic of the element. Thisspectrum is in essence a “fingerprint” of an element and offers a practicalmethod of identification for liquid media such as paint. Sodium vapor, forexample, always shows the same line spectrum, which differs from thespectrum of all other elements.

continuous spectrumA type of emission spectrum

showing a continuous band of

colors all blending into one

another.

emission spectrumLight emitted from a source

and separated into its

component colors or

frequencies.

line spectrumA type of emission spectrum

showing a series of lines

separated by black areas.

Each line represents a definite

wavelength or frequency.

Hydrogen

Helium

Mercury

FIGURE 11–9 Some characteristic emission spectra.

Trace Evidence II: Metals, Paint, and Soil 401

An emission spectrograph is an instrumentused to obtain and record the line spectraof elements. This instrument requires ameans for vaporizing and exciting theatoms of elements so that they emit light,a means for separating this light into itscomponent frequencies, and a means ofrecording the resultant spectrum. A simpleemission spectrograph is depicted inFigure 1.

The specimen under investigation isinserted between two carbon electrodesthrough which a direct current arc is passed.The arc produces enough heat to vaporizeand excite the specimen’s atoms. A lenscollects the light emitted by the excitedatoms and focuses it onto a prism thatdisperses it into component frequencies.

The Carbon Arc Emission Spectrograph

Closer Analysis

The separated frequencies of light are thendirected toward a photographic plate, wherethey are recorded as line images. Normally,a specimen consists of numerous elements;hence, the typical emission spectrumcontains many lines.

Each element in the spectrum can beidentified when it is compared to astandard chart that shows the position ofthe principal spectral lines of all theelements. However, forensic analysisusually requires a rapid comparison of theelemental composition of two or morespecimens. This is easily accomplished bycomparing the emission spectra line forline, as illustrated in Figure 2, in which theemission spectra of two paint chips areshown to be comparable.

Lens

Prism Photographic

plate

Sample

between

carbon

electrodes

FIGURE 1 Parts of a simple emission spectrograph.

FIGURE 2 A comparison of paint chips 1 and 2 by emission spectrographic analysis.A line-for-line comparison shows that the paints have the same elemental composition.

402 CHAPTER 11

To explain the origin of atomic spectra, wemust focus on the electron orbitals of theatom. As electrons move around thenucleus, they are confined to a path fromwhich they cannot stray. This orbital path isassociated with a definite amount of energyand is therefore called an energy level. Eachelement has its own set of characteristicenergy levels at varying distances from thenucleus. Some levels are occupied byelectrons; others are empty.

An atom is in its most stable state whenall of its electrons are positioned in theirlowest possible energy orbitals. When anatom absorbs energy, such as heat orlight, its electrons are pushed into higher-energy orbitals. In this condition, the atomis in an excited state. However, becauseenergy levels have fixed values, only adefinite amount of energy can be absorbedin moving an electron from one level toanother. This is an important observation;atoms absorb only a definite value ofenergy, and all other energy values areexcluded, as shown in the figure.

A specific frequency of light is required tocause this transition, and its energy mustcorrespond to the exact energy differencebetween the two orbitals involved. Thisenergy difference is expressed by therelationship E � hf, where E represents theenergy difference between the two orbitals, fis the frequency of absorbed light, and h is auniversal constant called Planck’s constant.Any energy value that is more or less than

The Origin of Emission Spectra

Closer Analysis

this difference will not produce thetransition. Hence, an element is selective inthe frequency of light it will absorb, and thisselectivity is determined by the electronenergy levels each element possesses.

In the same manner, if atoms are exposedto intense heat, enough energy will begenerated to push electrons intounoccupied higher-energy orbitals.Normally the electron does not remain inthis excited state for long, and it quicklyfalls back to its original energy level. Asthe electron falls back, it releases energy.An emission spectrum shows that thisenergy loss comes about in the form oflight emission (see figure). The frequencyof light emitted is again determined by therelationship E � hf, where E is the energydifference between the upper and lowerenergy levels and f is the frequency ofemitted light. Because each element hasits own characteristic set of energy levels,each emits a unique set of frequencyvalues. The emission spectrum thusprovides a “picture” of the energy levelsthat surround the nucleus of eachelement.

Thus, we see that as far as atoms areconcerned, energy is a two-way street.Energy can be put into the atom at thesame time that energy is given off; whatgoes in must come out. The chemist canstudy the atom using either approach.

(a) (b)

(a) The absorption of light byan atom, causing an electronto jump into a higher orbital.(b) The emission of light byan atom, caused by anelectron falling back to alower orbital.

excited stateThe state in which an

atom absorbs energy and

an electron moves from a

lower to a higher energy

level.

electron orbitalThe path of electrons as

they move around the

nuclei of atoms; each

orbital is associated with

a particular electronic

energy level.

Trace Evidence II: Metals, Paint, and Soil 403

Inductively Coupled Plasma Emission Spectrometry (ICP) Recently, induc-tively coupled plasma (ICP) emission spectrometry has supplanted carbonarc emission spectroscopy for most applications. Like emission spec-troscopy, ICP identifies and measures elements through light energy emit-ted by excited atoms. However, instead of using an electrical arc, the atomsare excited by placing the sample in a hot plasma torch. The torch is de-signed as three concentric quartz tubes through which argon gas flows(see Figure 11–10). A radio-frequency (RF) coil that carries a current iswrapped around the tubes. The RF current creates an intense magneticfield.

The process begins when a high-voltage spark is applied to the argongas flowing through the torch. This strips some electrons from their ar-gon atoms. These electrons are then caught and accelerated in the mag-netic field such that they collide with other argon atoms, stripping off stillmore electrons. The collision of electrons and argon atoms continues in achain reaction, breaking down the gas into argon atoms, argon ions, andelectrons, forming an inductively coupled plasma discharge. The dis-charge is sustained by RF energy that is continuously transferred to itfrom the coil.

The plasma discharge acts like a very intense continuous flame gener-ating temperatures in the range of 7,000–10,000°C. The sample, in aerosolform, is then introduced into the hot plasma, where it collides with the en-ergetic argon electrons and generates charged particles (ions). The ionsemit light of characteristic wavelengths that correspond to the identity ofthe elements in the sample.

The Significance of Paint EvidenceOnce a paint comparison is completed, the task of assessing the signifi-cance of the finding begins. How certain can one be that two similar paintscame from the same surface? For instance, a casual observer sees countlessidentically colored automobiles on our roads and streets. If this is the case,what value is a comparison of a paint chip from a hit-and-run scene to paintremoved from a suspect car?

From previous discussions it should be apparent that far more isinvolved in paint comparison than matching surface paint colors. Paintlayers beneath a surface layer offer valuable points of comparison. Fur-thermore, forensic analysts can detect subtle differences in paint binder

Sample

aerosol

Coil

Ions

++ ++

++

+++

++ +

Rf

generator

Plasma

discharge

FIGURE 11–10 The creation of charged particles in the torch of an ICP discharge.

404 CHAPTER 11

formulations, as well as major or minor differences in the elemental com-position of paint. Obviously, these properties cannot be discerned by thenaked eye.

The significance of a paint comparison was convincingly demonstratedfrom data gathered at the Centre of Forensic Science, Toronto, Canada.3

Paint chips randomly taken from 260 vehicles located in a local wreck yardwere compared by color, layer structure, and, when required, by infraredspectroscopy. All were distinguishable except for one pair. In statisticalterms, these results signify that if a crime-scene paint sample and a paintstandard/reference sample removed from a suspect car compare by thepreviously discussed tests, the odds against the crime-scene paint origi-nating from another randomly chosen vehicle are approximately 33,000 toone. Obviously, this type of evidence is bound to forge a strong link be-tween the suspect car and the crime scene.

Crime laboratories are often asked to identify the make and model ofa car from a very small amount of paint left behind at a crime scene. Suchinformation is frequently of use in a search for an unknown car involvedin a hit-and-run incident. Often the questioned paint can be identifiedwhen its color is compared to color chips representing the various makesand models of manufactured cars. However, in many cases it is notpossible to state the exact make or model of the car in question, becauseany one paint color can be found on more than one car model. For

Mutilated bullets often are not suitable fortraditional microscopic comparisonsagainst an exemplar test-fired bullet. Insuch situations, ICP has been used toobtain an elemental profile of thequestioned bullet fragment for comparisonagainst an unfired bullet generally found inthe possession of the suspect.

For a number of years forensic scientistshave been aware of significantcompositional differences among leadsources for the manufacture of lead-basedbullets. Knowledge of these differencescan be valuable when comparing bulletsthat are too mutilated to analyze with amicroscope. Compositional differences intrace elements are typically reflected in thecopper, arsenic, silver, antimony, bismuth,cadmium, and tin profiles of lead bullets.

When two or more bullets have comparableelemental compositions, evidence of theirsimilarity may be offered in a court of law. Inthis respect, the comparison of lead bullets

ICP Analysis of Bullets

Closer Analysis

faces the same quandry as most commontypes of class physical evidence. Theforensic analyst must convince a jury thatthe test results are meaningful to a criminalinquiry in the absence of any supportingstatistical or probability data. Furthermore,creating meaningful databases to define thestatistical significance of bullets comparedby elemental profiles is currently anunrealistic undertaking.

Nevertheless, the significant diversity ofbullet lead compositions in our population,like other class evidence such as fibers,hairs, paint, plastics, and glass, makestheir chance occurrence at a crime sceneand subsequent link to a defendant highlyunlikely. However, care must be taken toavoid giving the impression that elementalprofiles constitute a definitive match.Given the millions of bullets produced eachyear, one cannot conclusively rule out thepossibility of a coincidental match with anon-case-related bullet.

Trace Evidence II: Metals, Paint, and Soil 405

instance, General Motors may have used the same paint color for severalproduction years on cars in its Cadillac, Buick, Pontiac, and Chevroletlines.

Color charts for automobile finishes are available from various paintmanufacturers and refinishers (see Figure 11–11). Since 1975, the RoyalCanadian Mounted Police Forensic Laboratories have been systematicallygathering color and chemical information on automotive paints. This com-puterized database, known as PDQ (Paint Data Query), allows an analyst toobtain information on paints related to automobile make, model, and year.The database contains such parameters as automotive paint layer colors,primer colors, and binder composition (see Figure 11–12). A number ofU.S. laboratories have access to PDQ.4 Also, some crime laboratoriesmaintain an in-house collection of automotive paints associated with vari-ous makes and models, as shown in Figure 11–13.

Collection and Preservation of Paint EvidenceAs has already been noted, paint chips are most likely to be found on ornear people or objects involved in hit-and-run incidents. The recovery ofloose paint chips from a garment or from the road surface must be donewith the utmost care to keep the paint chip intact. Paint chips may be

FIGURE 11–11 Automotivecolor chart of various carmodels. Courtesy DamianDovanganes, AP Wide WorldPhotos

406 CHAPTER 11

FIGURE 11–12 (a) Home screen for PDQ database. (b) Partial list of autopaints containedin the PDQ database. Courtesy Royal Canadian Mounted Police

picked up with a tweezers or scooped up with a piece of paper. Paper drug-gist folds and glass or plastic vials make excellent containers for paint. Ifthe paint is smeared on or embedded in garments or objects, the investi-gator should not attempt to remove it; instead, it is best to package thewhole item carefully and send it to the laboratory for examination.

When a transfer of paint occurs in hit-and-run situations (such as tothe clothing of a pedestrian victim), uncontaminated standard/reference

(a)

(b)

Trace Evidence II: Metals, Paint, and Soil 407

paint must always be collected from an undamaged area of the vehiclefor comparison in the laboratory. The collected paint must be close to thearea of the car that is suspected of being in contact with the victim. Thisis necessary because other portions of the car may have faded or beenrepainted.

Standard/reference samples are always removed so as to include all thepaint layers down to the bare metal. This is best accomplished by removinga painted section with a clean scalpel or knife blade. Samples 1/4 inch squareare sufficient for laboratory examination. Each paint sample should be sep-arately packaged and marked with the exact location of its recovery.

When a cross-transfer of paint occurs between two vehicles, again allof the layers, including the foreign as well as the underlying original paints,must be removed from each vehicle. A standard/reference sample from anadjacent undamaged area of each vehicle must also be taken in such cases.Carefully wipe the blade of any knife or scraping tool used before collect-ing each sample, to avoid cross-contamination of paints.

FIGURE 11–13 A crime laboratory’s automotive paint library. Paints were collected at anautomobile impound yard and then cataloged for rapid retrieval and examination. CourtesyGavin Edmonstone, Centre for Forensic Sciences, Toronto, Canada

September in Arizona is hot and dry, muchlike the rest of the year—but September1984 was a little different. Unusually heavyrains fell for two days, which must haveseemed fitting for the friends and family of8-year-old Vicki Lynn Hoskinson. Vicki went

The Predator

Case Study

missing on September 17 of that year, andher disappearance was investigated as akidnapping. A school-teacher who knew Vickiremembered a suspicious vehicle loiteringnear the school that day, and he happenedto jot down the license plate number. This

(continued)

408 CHAPTER 11

crucial tip led police to 28-year-old FrankAtwood, recently paroled from a Californiaprison. Police soon learned that Atwood hadbeen convicted for committing sex offensesand for kidnapping a boy. This galvanizedthe investigators, who realized Vicki couldbe at the mercy of a dangerous andperverse man.

The only evidence the police had to workwith was Vicki’s bike, which was foundabandoned in the middle of the street a fewblocks from her home. Police found scrapesfrom her bike pedal on the underside of thegravel pan on Atwood’s car, as well as pinkpaint apparently transferred from Vicki’sbike to Atwood’s front bumper. The policebelieved that Atwood deliberately struckVicki while she was riding her bicycle,knocking her to the ground.

The pink paint on Atwood’s bumper wasfirst looked at microscopically and thenexamined by pyrolysis gas chromatography,which entails heating the paint sample toextremely high temperature to vaporize andfragment the components of the paint. Thepyrolyzed sample, in the form of a gas, isthen pushed through a gaschromatographic column. By the time thepaint components have reached the end ofthe column, they have separated and eachchemical constituent is recorded. Thistechnique provides investigators with a“fingerprint” pattern of the paint sample,enabling them to compare this paint to anyother paint evidence. In this case, the pinkpaint on Atwood’s bumper matched thepaint from Vicki’s bicycle.

Further evidence proving Atwood’sinvolvement in the crime came when a

gouge on the surface of Vicki’s bicycle waschecked for its elemental composition withthe aid of a scanning electron microscope(SEM) in combination with an X-ray analyzer.Traces of nickel were found on the gouge’ssurface. The bumper of Atwood’s car wascoated with a thin film of nickel, providingsolid evidence of the cross-transfer of paintand metal between the bumper of FrankAtwood’s car and Vicki Lynn’s bicycle.

Vicki’s skeletal remains were discovered inthe desert, several miles away from herhome, in the spring of 1985. Positiveidentification was made using dentalrecords, but investigators wanted to see ifthe remains could help them determinehow long she had been dead. Atwood wasjailed on an unrelated charge three daysafter Vicki disappeared, so theapproximate date of death was veryimportant to proving his guilt.

Investigators found adipocere, a white,fatty residue produced duringdecomposition, inside Vicki’s skull. Thisprovided evidence that moisture waspresent around Vicki’s body after herdeath, which does not make senseconsidering she was found in the Arizonadesert! A check of the weather revealedthat there had been an unusual amount ofrainfall at only one time since Vicki waslast seen alive: a mere forty-eight hoursafter her disappearance. This put Vicki’sdeath squarely within Frank Atwood’sthree-day window of opportunity betweenher disappearance and his arrest. FrankAtwood was sentenced to death in 1987for the murder of Vicki Lynn Hoskinson. Heremains on death row awaiting execution.

Key Points

• Paint spread onto a surface dries into a hard film that is best describedas consisting of pigments and additives suspended in the binder.

• Questioned and known paint specimens are best compared side by sideunder a stereoscopic microscope for color, surface texture, and colorlayer sequence.

Case StudyThe Predator (continued )

Trace Evidence II: Metals, Paint, and Soil 409

• Pyrolysis gas chromatography and infrared spectrophotometry areused to distinguish most paint binder formulations.

• Emission spectroscopy and inductively coupled plasma are techniquesavailable for determining the elemental composition of paint pigments.

• PDQ (Paint Data Query) is a computerized database that allows an an-alyst to obtain information on paints related to automobile make,model, and year.

Forensic Analysis of SoilThere are many definitions for the term soil; however, for forensic pur-poses, soil may be thought of as any disintegrated surface material, naturalor and artificial, that lies on or near the earth’s surface. Therefore, forensicexamination of soil not only is concerned with analysis of naturally occur-ring rocks, minerals, vegetation, and animal matter; it also encompassesdetection of such manufactured objects as glass, paint chips, asphalt, brickfragments, and cinders, whose presence may impart soil with characteris-tics that make it unique to a particular location. When this material is col-lected accidentally or deliberately in a manner that associates it with acrime under investigation, it becomes valuable physical evidence.5

The Significance of Soil EvidenceThe value of soil as evidence rests with its prevalence at crime scenes andits transferability between the scene and the criminal. Thus, soil or driedmud found adhering to a suspect’s clothing or shoes or to an automobile,when compared to soil samples collected at the crime site, may link a sus-pect or object to the crime scene. As with most types of physical evidence,forensic soil analysis is comparative in nature; soil found in the possessionof the suspect must be carefully collected to be compared to soil samplingsfrom the crime scene and its vicinity.

However, one should not rule out the value of soil even if the site of thecrime has not been ascertained. For instance, small amounts of soil may befound on a person or object far from the actual site of a crime. A geologistwho knows the local geology may be able to use geological maps to directpolice to the general vicinity where the soil was originally picked up andthe crime committed.

Forensic Examination of SoilMost soils can be differentiated by their gross appearance. A side-by-sidevisual comparison of the color and texture of soil specimens is easy to per-form and provides a sensitive property for distinguishing soils that origi-nate from different locations. Soil is darker when it is wet; therefore, colorcomparisons must always be made when all the samples are dried underidentical laboratory conditions. It is estimated that there are nearly 1,100distinguishable soil colors; hence, color offers a logical first step in a foren-sic soil comparison.

Microscopic Examination of Soil Low-power microscopic examination ofsoil reveals the presence of plant and animal materials as well as of artifi-cial debris. Further high-power microscopic examination helps character-ize minerals and rocks in earth materials. Although this approach to

410 CHAPTER 11

forensic soil identification requires the expertise of an investigator trainedin geology, it can provide the most varied and significant points of com-parison between soil samples. Only by carefully examining and comparingthe minerals and rocks naturally present in soil can one take advantage ofthe large number of variations between soils and thus add to the evidentialvalue of a positive comparison.6

A mineral is a naturally occurring crystal, and like any other crystal, itsphysical properties—for example, color, geometric shape, density, and re-fractive index or birefringence—are useful for identification. More than2,200 minerals exist; however, most are so rare that forensic geologistsusually encounter only about twenty of the more common ones. Rocks arecomposed of a combination of minerals and therefore exist in thousands ofvarieties on the earth’s surface. They are usually identified by characteriz-ing their mineral content and grain size (see Figure 11–14).

Considering the vast variety of minerals and rocks and the possiblepresence of artificial debris in soil, the forensic geologist is presented withmany points of comparison between two or more specimens. The numberof comparative points and their frequency of occurrence must be consid-ered before concluding similarity between specimens and judging theprobability of common origin.

Rocks and minerals not only are present in earth materials but are alsoused to manufacture a wide variety of industrial and commercial products.For example, the tools and garments of an individual suspected of break-ing into a safe often contain traces of safe insulation. Safe insulation maybe made from a wide combination of mineral mixtures that provide signif-icant points of identification. Similarly, building materials such as brick,plaster, and concrete blocks are combinations of minerals and rocks thatcan easily be recognized and compared microscopically to similar miner-als found on the breaking-and-entering suspect.

mineralA naturally occurring

crystalline solid.

FIGURE 11–14 A mineral viewed under a microscope. Courtesy Chris Palenik, Ph. D.,Microtrace, Elgin, IL., www.microtracescientific.com

Trace Evidence II: Metals, Paint, and Soil 411

Density-Gradient Tube Some forensic laboratories utilize the density-gradient tube technique to compare soil specimens. Typically, glass tubes6–10 millimeters in diameter and 25–40 centimeters long are filled with lay-ers of two liquids mixed in varying proportions so that each layer has a dif-ferent density value. For example, tetrabromoethane (density 2.96 g/mL)and ethanol (density 0.789 g/mL) may be mixed so that each successivelayer has a lower density than the preceding one, from the bottom to thetop of the tube. The simplest gradient tube may have from six to ten layers,in which the bottom layer is pure tetrabromoethane and the top layer ispure ethanol, with corresponding variations of concentration in the layersbetween these two extremes.

When soil is added to the density-gradient tube, its particles sink to theportion of the tube that has a density of equal value; the particles remainsuspended in the liquid at this point. In this way, a density distribution pat-tern of soil particles can be obtained and compared to other specimenstreated in a similar manner (see Figure 11–15).

Only a few crime laboratories use this procedure to compare soil evi-dence. There is evidence that the test is far from definitive, because manysoils collected from different locations yield similar density distributionpatterns.7 At best, the density-gradient test is useful for comparing soilswhen it is used in combination with other tests.

Variations in SoilThe ultimate forensic value of soil evidence depends on its variation at thecrime scene. If, for example, soil is indistinguishable for miles surroundingthe location of a crime, it will have limited value in associating soil foundon the suspect with that particular site. Significant conclusions relating asuspect to a particular location through a soil comparison may be madewhen variations in soil composition occur every 10–100 yards from thecrime site. However, even when such variations do exist, the forensicgeologist usually cannot individualize soil to any one location unless anunusual combination of rare minerals, rocks, or artificial debris can belocated.

density-gradient tubeA glass tube filled from bottom

to top with liquids of

successively lighter densities;

used to determine the density

distribution of soil.

FIGURE 11–15 A soil comparison by density gradient tubes.Courtesy Philadelphia Police Department Laboratory

412 CHAPTER 11

No statistically valid forensic studies have examined the variability ofsoil evidence. A study conducted in southern Ontario, Canada, seems to in-dicate that soil in that part of Canada shows extensive diversity. It esti-mated a probability of less than 1 in 50 of finding two soils that areindistinguishable in both color and mineral properties but originate in twodifferent locations separated by a distance of 1,000 feet. Based on thesepreliminary results, similar diversity may be expected in the northernUnited States, Canada, northern Europe, and eastern Europe. However,such probability values can only generally indicate the variation of soilwithin these geographical areas. Each crime scene must be evaluatedseparately to establish its own soil variation probabilities.

Collection and Preservation of Soil EvidenceWhen gathering soil specimens, the evidence collector must give primaryconsideration to establishing the variation of soil at the crime-scene area.For this reason, standard/reference soils should be collected at various in-tervals within a 100-yard radius of the crime scene, as well as at the site ofthe crime, for comparison to the questioned soil. Soil specimens alsoshould be collected at all possible alibi locations that the suspect may claim.

All specimens gathered should be representative of the soil that wasremoved by the suspect. In most cases, only the top layer of soil is pickedup during the commission of a crime. Thus, standard/reference specimensmust be removed from the surface without digging too deeply into the un-representative subsurface layers. Approximately a tablespoon or two ofsoil is all the laboratory needs for a thorough comparative analysis. Allspecimens collected should be packaged in individual containers, such asplastic vials. Each vial should be marked to indicate the location at whichthe sampling was made.

Soil found on a suspect must be carefully preserved for analysis. If it isfound adhering to an object, as in the case of soil on a shoe, the investiga-tor must not remove it. Instead, each object should be individually wrappedin paper, with the soil intact, and transmitted to the laboratory. Similarly,loose soil adhering to garments should not be removed; these items shouldbe carefully and individually wrapped in paper bags and sent to the labo-ratory for analysis. Care must be taken that particles that may accidentallyfall off the garment during transportation will remain in the paper bag.

When a lump of soil is found, it should be collected and preserved in-tact. For example, an automobile tends to collect and build up layers of soilunder the fenders, body, and so on. The impact of an automobile with an-other object may jar some of this soil loose. Once the suspect car has beenapprehended, a comparison of the soil left at the scene with soil remainingon the automobile may help establish that the car was present at the acci-dent scene. In these situations, separate samples are collected from underall the fender and frame areas of the vehicle; care is taken to remove thesoil in lump form in order to preserve the order in which the soil adheredto the car. Undoubtedly, during the normal use of an automobile, soil willbe picked up from numerous locations over a period of months and years.This layering effect may impart soil with greater variation, and hencegreater evidential value, than that normally associated with loose soil.

Key Points

• A side-by-side visual comparison of the color and texture of soil speci-mens provides a way to distinguish soils that originate from differentlocations.

Trace Evidence II: Metals, Paint, and Soil 413

• Minerals are naturally occurring crystalline solids found in soil. Theirphysical properties—for example, color, geometric shape, density, andrefractive index or birefringence—are useful for characterizing soils.

• Some crime laboratories compare soils by using density-gradient tubesfilled with layers of liquids with different density values.

Forensic Brief

Alice Redmond was reported missing byher husband on a Monday night in 1983.Police learned that she had been seenwith a co-worker, Mark Miller, after workthat evening. When police questionedMiller, he stated that the two just “drovearound” after work and then she droppedhim off at home. Despite his statement,Miller was the prime suspect because hehad a criminal record for burglary andtheft.

Alice’s car was recovered in town thefollowing morning. The wheel wells werethickly coated in mud, which investigatorshoped might provide a good lead. Thesehopes were dampened when policelearned that Alice and her husband hadattended a motorcycle race on Sunday,where her car was driven through deep mud.

After careful scrutiny, analysts found twocolors of soil on the undercarriage ofAlice’s car. The thickest soil was brown; ontop of the brown layer was a reddish soilthat looked unlike anything in the county.Investigators hoped the reddish soil, whichhad to have been deposited sometimeafter the Sunday night motorcycle eventand before the vehicle was discovered onTuesday morning, could link the vehicle tothe location of Alice Redmond.

An interview with Mark Miller’s sisterprovided a break in the case. She toldpolice that Mark had visited her on Monday

evening. During that visit, he confessedthat he had driven Alice in her car acrossthe Alabama state line into Georgia, killedher, and buried her in a remote location.Now that investigators had a better ideawhere to look for Alice, forensic analyststook soil samples that would prove ordisprove Miller’s sister’s story.

Each field sample was dried and comparedfor color and texture by eye andstereomicroscopy to the reddish-coloredsoil gathered from the car. Next, soils thatcompared to the car were passed througha series of mesh filters, each of a finergauge than the last. In this way, thecomponents of the soil samples werephysically separated by size. Finally, eachfraction was analyzed and compared formineral composition with the aid of apolarizing light microscope.

Only samples collected from areas acrossthe Alabama state line near the suspecteddump site were consistent with thetopmost reddish soil recovered fromAlice’s car. This finding supported Miller’ssister’s story and was instrumental inMark Miller’s being charged with murderand kidnapping. After pleading guilty, thedefendant led the authorities to where hehad buried the body. The burial site waswithin a half mile of the location whereforensic analysts had collected a soilsample consistent with the soil removedfrom Alice’s vehicle.

Source: T. J. Hopen, “The Value of Soil Evidence,” in Trace Evidence Analysis: More Cases in MuteWitnesses, M. M. Houck, ed. (Elsevier Academic Press, Burlington, Mass.:, 2004), pp. 105–122.

Soil: The Silent Witness

414 CHAPTER 11

Chapter SummaryFor the criminalist, the presence of trace elements is particularly useful,because they provide “invisible” markers that may establish the source ofa material or at least provide additional points for comparison.

A popular descriptive model of the atom pictures an atom as consist-ing of electrons orbiting a central nucleus. The nucleus is composed of pos-itively charged protons and neutrons that have no charge. Because theatom has no net electrical charge, the number of protons must alwaysequal the number of electrons in orbit around the nucleus.

One method used to identify trace elements in metals is neutron acti-vation analysis, which measures the gamma-ray frequencies of specimensthat have been bombarded with neutrons. The nucleus of an atom capturesone or more neutrons and forms an isotope and often decomposes, emit-ting radioactivity in the form of gamma-ray frequencies. This method pro-vides a highly sensitive and nondestructive analysis for simultaneouslyidentifying and quantitating twenty to thirty trace elements. Because thistechnique requires access to a nuclear reactor, however, it has limited valueto forensic analysis.

Paint spread onto a surface dries into a hard film consisting of pig-ments and additives suspended in the binder. One of the most commontypes of paint examined in the crime laboratory is finishes from automobiles.Manufacturers apply a variety of coatings to the body of an automobile.Hence, the wide diversity of automotive paint contributes to the forensicsignificance of an automobile paint comparison.

Questioned and known paint specimens are best compared side by sideunder a stereoscopic microscope for color, surface texture, and color layersequence. Pyrolysis gas chromatography and infrared spectrophotometryare invaluable for distinguishing most paint formulations, adding furthersignificance to a forensic paint comparison.

Emission spectroscopy and inductively coupled plasma are two tech-niques available to forensic scientists for determining the elementalcomposition of paint and other substances. An emission spectrographvaporizes and heats samples to a high temperature so that the atoms in thematerial achieve an “excited” state. Under these circumstances, the excitedatoms emit light. If the light is separated into its components, one observesa line spectrum. Each element in the spectrum can be identified by its char-acteristic line frequencies. In inductively coupled plasma, the sample, in theform of an aerosol, is introduced into a hot plasma, creating charged par-ticles that emit light of characteristic wavelengths corresponding to theidentity of the elements present.

The value of soil as evidence rests with its prevalence at crime scenesand its transferability between the scene and the criminal. Most soils canbe differentiated by their gross appearance. A side-by-side visual compar-ison of the color and texture of soil specimens is easy to perform andprovides a sensitive property for distinguishing soils that originate fromdifferent locations. In many forensic laboratories, forensic geologists char-acterize and compare the mineral content of soils. Some crime laboratoriesuse density-gradient tubes to compare soils. These tubes are typically filledwith layers of liquids that have different density values.

Trace Evidence II: Metals, Paint, and Soil 415

Review QuestionsFacts and Concepts

1. What are trace elements and how are they useful to forensic scientists?

2. Forensic analysis of the bullets recovered after the assassination of PresidentJohn F. Kennedy focused on the concentration of what two trace elements?

3. Which of the following conclusions can be drawn from the forensic analysisof the bullets recovered after the assassination of President John F. Kennedy?

a. The analysis absolutely verified the findings of the Warren Commission.b. The analysis cast doubt on the findings of the Warren Commission.c. The analysis generally supported the findings of the Warren Commission.d. The analysis absolutely disproved the findings of the Warren Commission.

4. Name the three most important subatomic particles and their electricalcharges, and indicate where each is located in the atom.

5. How does atomic number differ from atomic mass?

6. What is an isotope?

7. What is radioactivity? What are the three types of radiation?

8. Briefly describe the process of neutron activation analysis.

9. Name two advantages and two drawbacks of neutron activation analysis.

10. In what types of criminal cases is paint evidence most frequently encountered?

11. Describe the basic composition of paint. What component of paint evaporatesafter paint is applied to a surface?

12. What aspect of the automotive painting process is helpful in forensic charac-terization of automobile paint? How is it helpful?

13. What is the best way to make a microscopic comparison of paint chips?

14. What characteristics does a criminalist look for when comparing paint chipsunder a microscope? Which of these characteristics is most important in eval-uating the significance of paint evidence?

15. A thorough comparison of paint must include a chemical analysis of what twoproperties?

16. What chromatographic process has proven to be particularly invaluable fordistinguishing most paint formulations? Why is it so useful in the comparisonof paint?

17. Briefly describe the process of emission spectroscopy.

18. What technique has largely supplanted carbon arc emission spectroscopy formost applications? What is the main difference between the two techniques?

19. What is PDQ and how is it helpful to forensic scientists?

20. Why must paint collected from a vehicle involved in a hit-and-run accident betaken from the area of the car suspected of being in contact with the victim?

21. How should the investigator handle the collection of trace paint evidence lefton a tool?

416 CHAPTER 11

22. What is the first step in a forensic soil comparison?

23. What is a mineral? How are minerals useful in forensic soil analysis?

24. Briefly describe how soil is separated in a density-gradient tube.

25. From what areas should standard/reference soils be collected when gather-ing soil evidence?

26. How should soil evidence adhering to shoes and clothing be collected andpackaged?

27. How should lumps of soil be collected and preserved? Why?

Application and Critical Thinking1. Using the periodic table shown in Chapter 4, determine the atomic numbers for

antimony, tin, barium, and lead. Which do you think has the largest atomicmass? Which do you think has the smallest atomic mass? Explain your answers.

2. You are investigating a hit-and-run accident and have identified a suspect ve-hicle. Describe how you would collect paint to determine whether the suspectvehicle was involved in the accident. Be sure to indicate the tools you woulduse and the steps you would take to prevent cross contamination.

3. Criminalist Jared Heath responds to the scene of an assault on an unpavedlane in a rural neighborhood. Rain had fallen steadily the night before, mak-ing the area quite muddy. A suspect with very muddy shoes was apprehendednearby, but claimed to have picked up the mud either from his garden or fromthe unpaved parking lot of a local restaurant. Jared uses a spade to removeseveral samples of soil, each about 2 inches deep, from the immediate crimescene, and places each in a separate plastic vial. He collects the muddy shoesand wraps them in plastic as well. At the laboratory, he unpackages the soilsamples and examines them carefully, one at a time. He then analyzes the soilon the shoes to see if it matches the soil from the crime scene. What mistakes,if any, did Jared make in his investigation?

Case AnalysisIn the case of the CBS Murders, police suspected that the female victim had beenmoved from the location where she was murdered. Although an eyewitness placeda woman at the scene of the shooting, trace evidence ultimately was the key to solv-ing the case.

1. What was the primary challenge facing the investigators in this case?

2. What items of evidence directly linked the victim to the suspect? Which itemsindirectly linked the two?

3. How was paint evidence used to show that the female victim was shot at thegarage?

Trace Evidence II: Metals, Paint, and Soil 417

Web ResourcesForensic Paint Analysis and Comparison Guidelines (Article from the July 1999 issueof the online FBI journal Forensic Science Communications)www.fbi.gov/hq/lab/fsc/backissu/july1999/painta.htm

Collecting Crime Evidence from Earth (Online article by forensic geologist RaymondC. Murray that includes a discussion of soil analysis)www.forensicgeology.net/science.htm

Forensic Examination of Soil Evidence (Online report from Interpol aboutmethods/advances in soil examination)www.interpol.int/Public/Forensic/IFSS/meeting13/Reviews/Soil.pdf

The Atom Builder (Interactive exercise that explains fundamentals of atomic structureand allows users to try to construct stable atoms from subatomic particles)www.pbs.org/wgbh/aso/tryit/atom/#

The Atom and Electromagnetic Radiation (Discussion of nuclear structure andelectromagnetic radiation with practice problems)chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/atom_emrframe.html

Intro to Quantum Mechanics (Layperson’s explanation of basic concepts of quantumtheory)www.hi.is/~hj/QuantumMechanics/quantum.html

418 CHAPTER 11

Endnotes1. Actually, the electrons are moving so rapidly around the nucleus that they are best

visualized as an electron cloud spread out over the surface of the atom.

2. P. G. Rodgers et al., “The Classification of Automobile Paint by Diamond WindowInfrared Spectrophotometry, Part I: Binders and Pigments,” Canadian Society ofForensic Science Journal 9 (1976): 1; T. J. Allen, “Paint Sample Presentation forFourier Transform Infrared Microscopy,” Vibration Spectroscopy 3 (1992): 217.

3. G. Edmondstone, J. Hellman, K. Legate, G. L. Vardy, and E. Lindsay, “AnAssessment of the Evidential Value of Automotive Paint Comparisons,” CanadianSociety of Forensic Science Journal 37 (2004): 147.

4. J. L. Buckle et al., “PDQ—Paint Data Queries: The History and Technology behindthe Development of the Royal Canadian Mounted Police Laboratory ServicesAutomotive Paint Database,” Canadian Society of Forensic Science Journal 30(1997): 199. An excellent discussion of the PDQ database is also available in A.Beveridge, T. Fung, and D. MacDougall, “Use of Infrared Spectroscopy for theCharacterisation of Paint Fragments,” in B. Caddy, ed., Forensic Examination ofGlass and Paint (New York: Taylor & Francis, 2001), pp. 222–233.

5. E. P. Junger, “Assessing the Unique Characteristics of Close-Proximity SoilSamples: Just How Useful Is Soil Evidence?” Journal of Forensic Sciences 41(1996), 27.

6. W. J. Graves, “A Mineralogical Soil Classification Technique for the ForensicScientist,” Journal of Forensic Sciences 24 (1979): 323; M. J. McVicar and W. J.Graves, “The Forensic Comparison of Soil by Automated Scanning ElectronMicroscopy,” Canadian Society of Forensic Science Journal 30 (1997): 241.

7. K. Chaperlin and P. S. Howarth, “Soil Comparison by the Density GradientMethod—A Review and Evaluation,” Forensic Science International 23 (1983):161–177.

Case ReadingThe CBS Murders*

In the early morning hours . . . atop alonely roof garage on the west side ofManhattan, three men [all employed byCBS-TV] were found murdered. Each manhad been shot once in the back of thehead. A light-colored van was seenspeeding away from the scene. Hourslater, in a secluded alley street on thelower east side of Manhattan, the body ofa fully clothed woman was found lying facedown by two dog walkers. The woman hadbeen killed in the same manner as themen on the roof garage. The condition ofthe woman’s body, and other evidence,made it apparent that she had been shotat the garage, and then transported to thealley.

An eyewitness to the incident stated thathe saw a man shoot a woman and placeher in a light-colored van. The gunman thenchased down the three men who werecoming to the woman’s aid, and shot eachone of them. Days later, the prime suspectto the killings was arrested in Kentucky, ina black-colored van.

Numerous items of evidence (over 100)were collected from the van, andforwarded to the New York City policelaboratory for examination. Among theitems of evidence forwarded were threesets of vacuum sweepings from the van’sinterior.

An autopsy of the woman produced severalitems of trace evidence that were removedfrom the victim and forwarded to theauthor for microscopic examination. Thewoman’s clothing was also received by theauthor for trace analysis.

A prime question that arose during theinvestigation was: could the woman’s

body, which had been placed in a light-colored van at the garage, and later left inan alley on the lower east side, beassociated with the black van recoveredover 1000 km (600 miles) away from thescene? Microscopic analysis andcomparison of the trace evidentialmaterials found on the victim and insidethe van made this association possible.

Listed in Table 1 are all the items ofsimilar trace materials that both the victimand the van had in common.

Microscopic comparisons of thequestioned human head hair present onthe victim’s clothing were made withknown samples. Ten of the brown-coloredand gray-colored Caucasian head hairsfrom the victim’s blazer were consistent inmicroscopic characteristics to thedefendant’s known head hair sample. Onechemically treated head hair found on thevictim was consistent in microscopiccharacteristics to the known head hairsample obtained from the defendant’swife. One forcibly removed, brown-colored,Caucasian head hair that was found on therear door of the van’s interior by theKentucky state police was found to beconsistent in all characteristics with thedecedent’s known head hair sample.

Microscopic comparisons of the white- andbrown/white-colored dog hair from thevictim’s clothing, and the van’s interior,were made with known samples of dog hairobtained from a dog owned by thedefendant’s nephew, the van’s previousowner. The questioned dog hairs werefound to be consistent with the hair fromthe nephew’s dog.

Source: Reprinted by permission of the AmericanSociety of Testing and Materials from N. Petraco,“Trace Evidence—The Invisible Witness,” Journalof Forensic Science 31 (1986), 321. Copyright1986.

* This case takes its title from the fact that thethree victims were employees of CBS-TV.

Trace Evidence II: Metals, Paint, and Soil 419

420 CHAPTER 11

Table 1 Items of Similar Trace Evidence That Were Recovered fromBoth the Victim and the Van’s Interior

Source

Trace Evidence Victim Van

White seed mouth sweepings

Paint chips hair and wool sweepings gray/metallic/ blazer and floor

black

Sawdust hair, blazer, and sweepings and sheet misc. items

Glass fragments wool blazer and sweepings and clear sheet misc. items

amber

green

Cellophane wool blazer floor

Urethane foam wool blazer sweepings, misc. foam mattress items, and foam

mattress

Blue olefin skirt floor

plastic

Dog hair wool blazer sweepings and misc. brown/white items

white

Human hair wool blazer hairbrush, sweepings, brown and misc. items

gray

The white seed that was recovered fromthe victim’s mouth by the medicalexaminer, and the white seed that wasfound in the van’s sweepings by theauthor, were forwarded to aninternationally known botanist foridentification and comparison. During thetrial, the botanist testified that the twoseeds were identical in all respects, andthat although he could not identify theseed, both were either from the samespecies of plant, if not the same plant,probably a rare wild flower.

Sixteen gray metallic/black-colored paintchips from the victim and her clothing were

compared to the gray metallic/black-colored paint removed from the van.Samples from the questioned and knownsources were examined and compared bymicroscopic, chemical, and instrumentalmeans. All of the paint specimens fromthe van and from the victim were found tobe similar in all respects.

The remaining items of trace evidence fromthe victim and the van were examined andcompared microscopically, and wherenecessary, by chemical and instrumentalmethods. Each of the remaining types oftrace evidence from the victim was found tobe similar to its counterpart from the van.

Trace Evidence II: Metals, Paint, and Soil 421

Blue- and black-colored flakes of acrylicpaint were found in the van’s sweepings,and on the suspect’s sneakers. No blue- orblack-colored paint flakes were found onthe victim and her clothing. During a crimescene search of the defendant’s residencein New Jersey, a large quantity of blue- andblack-colored acrylic paint was found in thegarage. It was apparent from the evidencepresent in the defendant’s garage that alarge rectangular shaped object hadrecently been painted with blue- and black-colored paint. The blue and black paintflakes from all the sources and the knownblue (undercoat) and black (topcoat) paintfrom the van were compared bymicroscopic, chemical, and instrumental

means. All the samples of paint werefound to be consistent in every respect.

At the trial, extensive testimony concerningthe collection, examination, identification,and comparison of the trace evidence fromthe victim and the van was given by theauthor, over a two-day period. Whenquestioned about the source of the traceevidence found on the victim and herclothing, the author stated unequivocallythat the trace evidence on the victim wasfrom the defendant’s van. On the basis ofthis evidence and other circumstantialevidence, the defendant was found guiltyof all charges and sentenced to 100 yearsin prison.